[Lil' Lavery]

Sense 2003 (my first year on 116), 116 has used pnuematics on 3/4 robots, this year being the only we didn't.
In 2003, we used pnuematics for shifting only.
In 2004, we used pnuematics for shifting, and our herding arms (later replaced by a pnuematic wheelie bar).
In 2005, we used pnuematics to extend a moment arm to make our winch expend less power to raise our arm (and later we added a pnuematic "claw" to keep tetras from falling off of our arm).
We had an on-board compressor for all 3 years.

Pnuematics are a tremendous asset for your robot, and are terrific for linear motion. They are quick, simple, and reliable, much more so than motors. While the compressor and air tanks may weigh a fair bit, you can position them near the base of your robot, therefore improving your CG and stability. The actually pistons are typically very lightweight, and won't hamper your total weight budget or center of gravity much. Motors can be quite weight, especially when you factor in gears, chains, pullies, and other mechanisms to adjust the motors speed and moving the force to other locations.
Motors have a greater degree of control though. In FIRST applications, pistons can either be extended or retracted, while motors can be anywhere in between.

I prefer to use pistons wherever you do not need huge degrees of control, as they are, in total, often lighter components than motors, and typically much simpler to design and constuct, recquire less space, help lower CG (usually), and are quicker at accomplishing functions.